Hodiaŭ en la oficejo ni havas longan kunvenon pri nova projekto.

Because kunvenon is the direct object of havas. In Esperanto, the direct object usually takes the accusative ending -n.

• ni havas kunvenon = we have a meeting
• kunvenon is what we have, so it gets -n

The adjective has to match the noun, so longa also becomes longan.

So:

• longa kunveno = a long meeting
• longan kunvenon = a long meeting, when it is the direct object

Because nova projekto comes after the preposition pri.

In Esperanto, a noun after a preposition normally does not take the accusative -n just because it follows that preposition. So:

• pri nova projekto = about a new project

Here, projekto is not the direct object of havas. The whole phrase pri nova projekto simply tells what the meeting is about.

Pri means about, concerning, or regarding.

So:

• kunveno pri nova projekto = a meeting about a new project

It is a very common preposition in Esperanto for topics, subjects, and matters under discussion.

Yes. Havi means to have, and Esperanto often uses havi kunvenon in the same natural way English says have a meeting.

So this is completely normal Esperanto:

• Ni havas kunvenon. = We have a meeting.

You could also say things in other ways depending on context, but havi kunvenon is standard and idiomatic.

In Esperanto, verbs do not change for person or number.

That means:

• mi havas = I have
• vi havas = you have
• li havas = he has
• ni havas = we have
• ili havas = they have

The ending -as always marks the present tense. The subject pronoun tells you who is doing the action.

Yes, the word order can be changed more freely than in English.

This sentence begins with Hodiaŭ and en la oficejo to set the scene first:

• Hodiaŭ = today
• en la oficejo = in the office

Then comes the main clause:

• ni havas longan kunvenon...

You could also say:

• Ni havas longan kunvenon hodiaŭ en la oficejo.

That is also correct. The original order just sounds natural and gives time and place first.

La is the definite article, meaning the.

So:

• en la oficejo = in the office

It usually suggests a specific office that speaker and listener know about, such as the office where they work.

If you said just en oficejo, that would sound unusual in most contexts, because Esperanto normally uses la when a place is understood to be specific.

Yes. Kunveno is built from familiar Esperanto parts:

• kun = with, together
• ven = come
• -o = noun ending

So the word has the idea of a coming together, which is why it means meeting or gathering.

This is a good example of how Esperanto often builds vocabulary from roots and affixes in a very transparent way.

Yes. Esperanto adjectives can come before or after the noun.

So both are grammatical:

• longan kunvenon
• kunvenon longan

But putting the adjective before the noun is usually more neutral and more common in everyday usage.

In either position, the adjective still has to agree with the noun, so it keeps the -n here.

Hodiaŭ is pronounced approximately ho-DEE-ow.

A few useful points:

• Esperanto stress is normally on the second-to-last syllable.
• hodiaŭ has three syllables: ho-di-aŭ
• The stress is on di

The letter ŭ is not a full vowel by itself. It appears in diphthongs such as aŭ and sounds like the w-glide in English now or cow.

So aŭ sounds roughly like ow in now.

Because all the nouns here are singular:

• la oficejo = the office
• longan kunvenon = a long meeting
• nova projekto = a new project

In Esperanto, plural nouns take -j:

• oficejoj = offices
• kunvenoj = meetings
• projektoj = projects

If the sentence were plural, you would also make the adjectives match:

• longaj kunvenoj
• novaj projektoj

But in your sentence, each of those nouns is singular, so there is no -j.